Polylactic acid resin composition and film

ABSTRACT

Provided is a polylactic acid resin composition useful as a molding material and a packaging material. The polylactic acid resin composition comprises a resin composition (l) comprising poly-L-lactic acid having an L-isomer content of 90-100 mol % as the main component and a resin composition (d) comprising poly-D-lactic acid having a D-isomer content of 90-100 mol % as the main component, the resin compositions (l) and (d) having been alternately stacked so that each layer has a thickness of 0.01-2.5 μm. The polylactic acid resin composition has specific thermal properties, can be used at high temperatures even after high-temperature melt molding, and can give molded polylactic acid articles having excellent suitability for various kinds of processing, such as printing, and having excellent transparency, etc. Also provided is a stretched polylactic acid resin film obtained by heating the polylactic acid resin composition to 280° C. or higher, subsequently cooling the composition, thereafter forming the polylactic acid resin composition into a film, and then stretching the film in at least one direction. This film can be used at high temperatures and has excellent suitability for various kinds of processing, excellent transparency, etc.

TECHNICAL FIELD

The present invention relates to a polylactic acid resin composition anda film thereof and provides a polylactic acid film that can be used athigher temperatures in comparison to a conventional polylactic acid filmand that is superior in suitability for various processing such asprinting, superior in transparency, etc., and useful not only as awrapping material but also as an industrial material.

BACKGROUND ART

For keeping up with recent environmental problems, various types ofmaterials have been studied. There are three strategies, i.e., (1)reduction in dependency on fossil resources, (2) biodegradability forreduction in environmental load, and (3) improvement in recyclability ofmaterials and easy regeneration of resources. Polylactic acid has beenstudied continuously over 20 years as a material that is derived from abiomass and has biodegradability. It has started to be used for fibers,films, containers and molding materials, but slow crystallization rate,low strength, low heat resistance, low barrier property, and poorerbalance between properties and price in comparison to general purposepetroleum resins have been pointed out as problems. In particular,improvement in properties, namely, heat resistance, solvent resistanceand strength, is important and various studies are being made atpresent.

As a most-widely known technology for improving properties themselves ofa resin, there is known a technology of forming a stereocomplex. Astereocomplex is a crystal in which a segment of a poly-L-lactic acid(hereinafter abbreviated as PLLA) and a segment of a poly-D-lactic acid(hereinafter abbreviated as PDLA) are packed in a one-to-one ratio andthis can increase the melting point of PLLA alone or PDLA alone by about50° C. In addition to that, it is known that mechanical properties,solvent resistance, and gas barrier property are improved bystereocomplex formation, and this is researched currently by manycompanies.

As to a method for producing a stereocomplex, it can be obtained byblending PLLA and PDLA, but there are the following problems withpractical production as is apparent from prior art documents.

(1) When using a high molecular weight polylactic acid, which isadvantageous in an aspect of mechanical properties, it is difficult toform a stereocomplex efficiently during the course of crystallization,and a large number of homocrystals of PLLA and PDLA, which are speciesto preferentially crystallize, and a small number of stereocomplexcrystals are formed by merely blending. Therefore, originally intendedimprovement in heat resistance and so on becomes insufficient.

(2) Although it has been known that the efficiency of stereocomplexformation is improved by kneading in a molten state, degradation of aresin caused by heat and fall of the melting point of a stereocomplexPLA due to transesterification between PLLA and PDLA also occur and,therefore, an effect sufficient for the intended goal can not beobtained.

With respect to these points, converting PLLA and PDLA into a blockpolymer (patent document 1), improving the efficiency of stereocomplexformation by reducing the molecular weight of one polylactic acid resin(patent document 2), forming a stereocomplex efficiently in a statewhere the molecular weight is low, and then performing solid phasepolymerization (patent document 3), copolymerizing another component toone polylactic acid in order to improve the compatibility with anotherpolylactic acid (patent document 4), and performing heat treatment at aspecified temperature for forming a stereocomplex (patent document 5)have been proposed for stereocomplex formation. However, these methodsare all not on the precondition of using a general purpose polylacticacid but on the precondition of improving a resin itself, and,therefore, there are at present many problems to be solved forindustrialization. Moreover, there have not been proposed any solutionsfor the problem that these methods are accompanied by atransesterification reaction in promoting stereocomplex formation or inmelt-molding a resulting resin composition again, resulting inoccurrence of fall of a melting point and so on.

Moreover, in the case of subjecting a general polyester resin or thelike to processing such as drawing, it is general to heat it to atemperature equal to or higher than its melting point to once melt itscrystals, rapidly cool the melt into an amorphous state, and process theamorphous within a temperature region of from the glass transition pointto the melting point. By a method in which a stereocomplex polylacticacid is heated once to about 250° C., which is a temperature higher thanthe melting point (140 to 170° C.) of a polylactic acid resin, therebymelting crystals of the polylactic acid and followed by rapidly coolingthe melt and the resulting molded article is subjected to drawing,rupture occurs due to fall of drawability caused by stereocomplexcrystals in a step of drawing or the like, and a drawn film was notobtained probably because the stereocomplex crystals in the resin cannot be melted completely. Then, it becomes possible to draw a moldedarticle by heating a stereocomplex polylactic acid to a temperature of280° C. or higher at which crystals of the stereocomplex polylactic acidcan melt completely; however, the melting point of the resulting resinbecomes a temperature that is far lower than the original melting pointof a stereocomplex crystal. This is probably because it has been pointedout that a stereocomplex is difficult to be formed again depending uponthe melting state of stereocomplex crystals which depends on thetemperature, the melting time and the like at the time of melting(non-patent document 1) and if stereocomplex crystals are melted once,homocrystals of PLLA and PDLA, which are species to preferentiallycrystallize, are formed earlier. As described above, there is at presentno method of industrially reconciling molding processability withefficiency of reformation of a stereocomplex polylactic acid crystalafter remelting.

Among the methods described in the above-listed patent documents, aconcrete production method is a method in which the preparation of astereocomplex is carried out by casting film from a solution. However,this method is not suitable for industrial mass production, or therehave been obtained only films resulting from pressing at temperatures aslow as about 250° C., or the melting point has been lowered or asufficient stereocomplex has not been obtained even in a productobtained by melting.

The above-described known technologies developed up to date aresummarized as follows.

(i) A stereocomplex body is difficult to be formed by only melt-blendingPLLA and PDLA. Moreover, a melting point falls due to atransesterification reaction if kneading PLLA and PDLA under melting inorder to promote stereocomplex formation. Even if a stereocomplex isformed, the stereocomplex is easily divided into PLLA and PDLA byremelting to each form a stereo single crystal, and a complex is hardlyformed again.

(ii) Reformation of a stereocomplex in remelting is promoted by methodssuch as block polymerization and reduction in the molecular weight ofone resin, but the fall of melting point due to a transesterificationreaction can not be suppressed.

PRIOR ART DOCUMENTS Patent Documents

-   Patent document 1: JP 2007-100104 A-   Patent document 2: JP 2003-96285 A-   Patent document 3: JP 2008-291268 A-   Patent document 4: JP 2007-191625 A-   Patent document 5: JP 2008-63356 A

Non-Patent Document

-   Non-patent document 1: Y. He et al./Polymer 49 (2008) 5670-5675    “Unique crystallization behavior of poly(L-lactide)/poly(D-lactide)    stereocomplex depending on initial melt states”

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The present invention intends to provide a polylactic acid resincomposition useful as a molding material or a wrapping material, whichcomposition can reform stereocomplex PLA more easily in comparison to aconventional composition even when producing a molded article bymelt-molding at a high temperature and can afford a polylactic acidmolded article the fall of the melting point of which caused by atransesterification reaction in remelting is suppressed. Moreover, itintends to provide a polylactic acid film that can be used at hightemperatures and is superior in suitability for various processing andin transparency.

Solutions to the Problems

The present inventors researched earnestly in order to solve theabove-mentioned problems, and have accomplished the present invention.The present inventors have found that it is possible to form astereocomplex efficiently by thinly dividing a layer of a PLLA melt anda layer of a PDLA layer, stacking many of such divided layersalternately to make the contact area of both resins large, and heatingthem. Moreover, they have found that the resulting polylactic acid has ahigh melting point and has a very high efficiency of reformingstereocomplex crystals even after melting at a conventionally-unexpectedhigh temperature of 280° C., and when the resulting polylactic acid isused for various types of melt-molding, it can afford polylactic acidmolded articles having various new superior properties. Thus, thepresent invention has been accomplished.

That is, the present invention is constituted as follows.

1. A polylactic acid resin composition, comprising a resin composition(l) primarily including a poly-L-lactic acid having an L-form content of90 to 100 mol % and a resin composition (d) primarily including apoly-D-lactic acid having a D-form content of 90 to 100 mol %, the resincompositions (l) and (d) being stacked alternately one on another with(l) and (d) each being 0.01 to 2.5 μm in a single layer thickness,wherein a peak of a melting point is observed at 210° C. or higher inDSC measurement using a 10 mg sample, and an enthalpy of fusion ismeasured to be 60 J/g or more when the sample is heated to 280° C., heldfor 3 minutes, then immediately cooled rapidly, and further subjected toDSC measurement at a temperature raising rate of 20° C. per minute.2. The polylactic acid resin composition according to 1, wherein theresin composition (l) and the resin composition (d) are stackedalternately one on another by making them pass a static mixer or amultilayer feed block.3. A polylactic acid resin drawn film produced by using the polylacticacid resin composition according to 1 or 2, heating it to 280° C. orhigher, then cooling it rapidly, and then drawing it in at least onedirection.

Effects of the Invention

The present invention is a polylactic acid resin composition that ishigh in melting point even after it is heated to 280° C. and melt-moldedand that can maintain a high stereocomplex formation efficiency aftermolding process; molded articles thereof exhibit not only superior heatresistance but also various properties as stereocomplex molded articles,such as superior mechanical properties, solvent resistance, and gasbarrier property. In particular, when it is processed into a film, afilm that particularly has various superior processing suitabilitiesderived from superior transparency and high heat resistance with a printpitch error suppressed is produced. In the method of the presentinvention, it is preferred to form stereocomplex PLLA and PDLA afterdividing them each into thin layers. By the use of this method, the timefor mixing PLLA with PDLA in a molten state for stereocomplex formationbecomes shorter than usual. It is conceivable that the above reduces thefall of the optical purities of the respective polylactic acid resins,and the fall of a melting point is reduced and a polylactic acid thatexhibits the aforementioned superior properties can be obtained by asimple process and a simple apparatus.

Although the fact that the efficiency of reforming stereocomplexcrystals is very high even after melting at a high temperature of 280°C., which is one feature of the present invention, is underconsideration, it is conceivable that by stacking PLLA and PDLA inlayers, a stereocomplex is formed at their interface, and since evenafter the stereocomplex is divided into PLLA and PDLA after remelting,these resins are arranged next to each other in layers, the contact areaof the resins is sufficiently large, single crystals of PLLA and PDLAare difficult to form, and, therefore, a stereocomplex is easily formed.It seems that a transesterification reaction is suppressed and the fallof the melting point of a stereocomplex is suppressed because astereocomplex is reformed easily and, therefore, excessive heating andso on are not needed.

MODE(S) FOR CARRYING OUT THE INVENTION

In the following, the present invention is described in detail.

(Polylactic Acid Resin)

The polylactic acid resin to be used in the present inventionessentially includes a poly-L-lactic acid resin (1) having an L-formcontent of 90 to 100% and a poly-D-lactic acid resin (d) having a D-formcontent of 90 to 100%. These resins can be obtained easily by melt ringopening polymerization from lactide, a raw material, as well as by usingcommercially available products. As a commercially available polylacticacid resin, Nature Works, which is an L-form polymer produced by NatureWorks, REVODE produced by Hisun Biomaterials Co., Ltd., Lacea, which isan L-form polymer produced by Mitsui Chemicals, Inc., and a polylacticacid resin produced by Purac can be used, and a polymer can be obtainedfrom raw materials using lactides produced by Musashino ChemicalLaboratory, Ltd. or Purac.

As to the molecular weight of the polylactic acid resin to be used inthe present invention, it is preferred that the polystyrene-equivalentweight average molecular weight of the above-mentioned (l) or (d) bewithin the range of from 100,000 to 300,000, more preferably from100,000 to 250,000, and even more preferably from 100,000 to 200,000.The case that both (l) and (d) are less than 100,000 is undesirablebecause if so, the resin becomes very brittle. The case that both (l)and (d) exceed 300,000 is undesirable because if so, the melt viscositybecomes so high that processability such as melt-moldabilitydeteriorates.

As to the thermal properties of the PLLA and the PDLA to be used in thepresent invention, their glass transition temperature is preferablywithin the range of from 40 to 70° C. If it is lower than 40° C., theoptical purity is low, which is not very preferable for stereocomplexformation. The melting point is preferably within the range of from 120to 170° C. If it is lower than 120° C., the optical purity is low, whichis not very preferable for stereocomplex formation.

To the layers primarily containing (l) and (d), respectively, may beadded various additives such as a lubricant, an antistatic agent, aconcealability imparting agent, a melt viscosity increase agent, anadhesiveness imparting agent, a barrier property imparting agent, anantioxidant, a thermal stability imparting agent, a plasticizer, and acrystal nucleating agent. Concretely, while silica, alumina, zirconia,magnesia, titania, salts of alkylsulfonic acids, polyfunctionalisocyanates, polyfunctional carbodiimide compounds, polyfunctional epoxycompounds, aliphatic and aromatic polyester resins, polyurethane resins,polycarbonate resins, acrylic resins, vinyl resins, low molecular weightbarrier property imparting agents, polyglycolic acid, antioxidants, suchas Irganox1010 (available from Ciba Japan), polyglycerols,polymerization catalyst deactivators such as phosphorus-containingcompounds, and so on can be listed, any substance other than inorganiccompounds must be 200° C. or lower in melting point. The case that themelting point exceeds 200° C. is undesirable because such a resin doesnot melt during the course of melting a polylactic acid resin, causingdefective appearance. The amount to be added of these additives, whichis adjusted appropriately for developing desired properties, ispreferably within the range of from 0 to 10% by weight. The case that itexceeds 10% by weight is undesirable because if so, various propertiesdeteriorate and stereocomplex formation is inhibited.

(Division and Stacking of Layers)

The most preferable method for obtaining a stereocomplex resin in thepresent invention is described below.

In order to obtain such a stereocomplex resin, many layers of resinlayers (L) and (D) made of PLLA or PDLA are stacked. As to a stackingmethod, a product can be obtained by a method including laminating resinsheets each composed of a resin layer (L) and a resin layer (D) andheating and rolling them, and a static mixer method or a multilayer feedblock method can be used. From an economic aspect, the latter methods,i.e., the static mixer method, the feed block method, or a combinationof both is preferred.

In the case of laminating sheets and heating and rolling them, thesheets before heating and rolling can be produced by ordinary methods,and concrete examples of such methods include a melt-extrusion method, amelt-pressing method, a calendering method, and a solution castingmethod. In the melt-extrusion method, the melting temperature ispreferably within the range of from 160 to 250° C. and more preferablywithin the range of from 170 to 240° C. That the melting temperature islower than 160° C. is undesirable because if so, the melt viscosity isexcessively high. That it is 250° C. or higher is undesirable because ifso, thermal decomposition of a polylactic acid resin or the like mayoccur. In solution casting, any solvent in which a polylactic acid resincan be dissolved is not particularly restricted. From the viewpoint ofproductivity, solvents having a boiling point of from 20 to 200° C. arepreferred, and examples thereof include chloroform andtetrachloromethane. Since the solution casting method allows a solventto remain and, therefore, its application may be restricted, methodsfailing to include any solvent, such as the melting method and thecalendering method, are preferred. The thickness of a sheet beforelaminating is preferably within the range of form 0.1 to 100 μm, andmore preferably within the range of from 1 to 10 μm. If it is 0.1 μm orless, handling becomes difficult due to wrinkling, and so on. The caseof being 100 μm or more is undesirable because if so, the overallthickness becomes excessively large in laminating into multiple layers,resulting in poor economical efficiency. As to the resulting sheet,sheets of PLLA and sheets of PDLA are laminated alternately, followed byraising the temperature of the sheets to their melting points or higherby heating, and then rolled. The thickness of the sheet before rollingand the number of layers to be laminated at the time of rolling arepreferably determined in consideration of the thickness of a layer atthe time of performing stereocomplex formation and the thickness of asheet after rolling. The thickness after rolling is preferably withinthe range of from 1 to 1000 μm. In one concrete example, five 100-μmthick sheets of PLLA and five 100-μm thick sheets of PDLA are prepared,and these are rolled with a pressing machine or the like at 180° C. withthe sheets laminated alternately, thereby forming a 100-μm thick sheet.In a similar way, ten rolled sheets in total are produced and the tensheets are laminated and rolled in a similar way except for adjustingthe temperature to 220° C. Thereby a laminated sheet having a layerthickness of 1 μm can be produced. When subjecting the resulting sheetto processing such as drawing during the following process, it isconceivable that a sheet in a state like a stereocomplex precursor thatcontains only a small amount of stereocomplex crystals but can form astereocomplex easily by heating can be obtained by adjusting thepressing temperature in a final pressing step to 260° C. or higher,preferably 270° C. or higher, more preferably 275° C. or higher, andparticularly preferably 280° C. or higher, and then cooling the sheetrapidly. Although the reason for this is unknown, the fact that crystalsof PLLA and PDLA, which are easily formed by nature, are prevented frombeing formed structurally due to PLLA and PDLA being stacked in athickness direction is presumed to cause that. The upper limit of thepressing temperature is preferably 350° C., and more preferably 330° C.or lower. That it exceeds 350° C. is undesirable because if so, thermaldegradation such as decomposition easily occurs. Rapid cooling in thepresent invention refers to cool a melt while maintaining its degree ofcrystallinity at 30% or lower during cooling the melt to a temperatureequal to or lower than its glass transition temperature, and concreteexamples thereof include, but are not limited to, performing cooling ata cooling rate of 200° C./min or more by immersion into liquid nitrogenor pressure bonding onto a cooling roll.

Similar preparation can be done by using a static mixer or a multilayerfeed block. Concretely, PLLA and PDLA are fed to two extruders to melt,and then a long sheet with a multilayer structure in which PLLA and PDLAare stacked alternately is produced easily in a short time by using astatic mixer or a multilayer feed block. Advantages of these methodsinclude that a long sheet can be obtained, that it is easy to controlthe weight ratio of the resins, and that the melting point is lesslowered in comparison to ordinary blending. With regard to this, whenpellets of ordinary PLLA and PDLA are blended and fed to an extruder, atransesterification reaction occurs and a phenomenon that the meltingpoint of the resulting stereocomplex falls is observed. Particularly,this phenomenon is observed when fully performing kneading in a moltenstate in order to improve the efficiency of stereocomplex formation, andthe improvement in the efficiency of stereocomplex formation and thefall of the melting point of the resulting stereocomplex body are in atradeoff relationship. On the other hand, in the case of stereocomplexformation in multilayering, PLLA and PDLA are brought into contact witheach other in a high surface area for a short time and, therefore, thetransesterification reaction of both the resins is minimized.

The resin compositions constituting the polylactic acid resins (l) and(d) (if necessary, other additives) are each fed to separate extrudersand extruded at temperatures equal to or higher than their meltingtemperatures. Concretely, the range of from 140 to 250° C. is preferred.The melting temperature is preferably up to a temperature 5° C. lowerthan the decomposition onset temperature. In particular, polylactic acidis liable to decomposition at 250° C. or higher and, therefore, it isnecessary to pay attention.

In the present invention, it is preferred that the aforementioned twomelts be divided respectively into layers having a thickness of 0.01 to2.5 μm by using a static mixer or a multilayer feed block and then thelayers be stacked one on another alternately. It is important, from theviewpoint of increasing stereocomplex formation efficiency, to dividelayers for increasing the contact area of both the resins. The thicknessof a layer to be formed by division is preferably 0.02 to 2 μm, and morepreferably 0.02 to 1 μm. An attempt to make the layer thinner than 0.01μm is undesirable because this is substantially equivalent to that PLLAand PDLA are melt-mixed and the melting point of a stereocomplex falls.That the layer is thicker than 2.5 μm is undesirable because thestereocomplex formation efficiency is poor and crystals of only PLLA andonly PDLA are easily formed.

As to the resin layers one of which primarily contains the polylacticacid resin (1) and the other primarily contains the polylactic acid (d),the melting temperature difference at the time of their lamination ispreferably 0 to 30° C., more preferably 0 to 20° C., and even morepreferably 0 to 10° C. When the melt viscosity difference in stackinglayers is up to 30 times, preferably up to 20 times, and more preferablyup to 10 times expressed by a shear rate estimated in a die, it becomespossible to suppress unevenness of appearance at the time of stackinglayers. In controlling the melt viscosity, the aforementionedmultifunctional compounds can be added. The feed block temperature ispreferably within the range of from 160 to 300° C., more preferably from170 to 290° C., and even more preferably from 180 to 280° C. When thefeed block temperature is low, the melt viscosity becomes excessivelyhigh and, therefore, the load applied to the extruder becomesexcessively high, whereas that the temperature is high is undesirable inan aspect of stereocomplex formation because if so, the melt viscositydifference becomes large and, therefore, unevenness or the like occurs.

The polylactic acid resin of the present invention is preferablysubjected to heat treatment after being stacked by the above-describedmethod. In this case, it is possible to heat a part of later stageelements of a static mixer, or extend a melt line after stacking layersby a feed block system and then perform heat treatment in a part or thewhole part of the melt line, or perform heat treatment in a partextending from a feed block to a die. The temperature of the heattreatment is preferably within the range of from 230 to 300° C., morepreferably from 235 to 295° C., and even more preferably from 235 to290° C. In a state of having been extruded through a die, the smallerthe content of stereocomplex crystals, the better it is. This can beconfirmed by using DSC from the amount of enthalpy of fusion of astereocomplex crystal. Preferably, the resulting stacked body ispelletized and then subjected to dry and crystallization. That is, itcan be said that one desirable embodiment of the polylactic acidcomposition of the present invention is a pellet form. Moreover,performing various types of molding directly without doing cooling, andforming a film in a manner described later after rapidly cooling andcasting by a T-shaped die method are included in preferred embodiments.

In the production of the polylactic acid of the present invention, toincrease the contact area of molten poly-L-lactic acid and moltenpoly-D-lactic acid rapidly without providing stirring as strong astransesterification occurs is efficient for increasing the stereocomplexformation efficiency.

Some of preferred production methods for the aforementioned purpose areas follows.

(1) A method in which polylactic acid resins (l) and (d) are broughtinto contact with each other efficiently by stacking molten polylacticacid resins (l) and (d) in two layers and repeating division andalternate stacking of them.

(2) A method in which melts in which molten polylactic acid resins (l)and (d) are bundled in a thin thread form are stacked as described aboveby using a die having such a structure that molten polylactic acidresins (l) and (d) are extruded separately through many small holes andthe small holes through which the polylactic acids (l) and (d) areextruded are combined alternately.

(3) A method in which a melt composed of the molten polylactic acids (l)and (d) described in the above (2) bundled into a thin thread form isintroduced into a thinning tube, thereby drawing it.

(4) A method in which polylactic acids (l) and (d) processed into afibrous form are mixed to bundle, and this is melted without stirring tostack or draw as described above.

(5) A method in which polylactic acids (l) and (d) are pulverizedseparately and dry-blended, and this is melted without stirring to stackor draw as described above.

The features of the stereocomplex polylactic acid of the presentinvention include that the stereocomplex body is stable even afterremelting it to a melting point 230° C. or higher and it hardly returnsto a blend of original PLLA and PDLA. Concretely, one feature is thatthe enthalpy of fusion measured at a temperature raising rate of 20°C./min in DSC is 60 J/g or more after heating the resultingstereocomplex body again to 280° C., holding it for 3 minutes, and thencooling it rapidly. For achieving such a feature, it is preferred toadjust the thickness of each layer within the range of from 0.01 to 2.5μm.

The ratio of the thickness of the layer made of the resin (1) to thethickness of the layer made of (d) in the present invention ispreferably from 70/30 to 30/70, and more preferably from 60/40 to 40/60.That one of the layers is thinner than 30% is undesirable because if so,an effect for improvement in heat resistance, which is an object of thepresent invention, is decreased.

(Glass Transition Temperature, Melting Point, Crystal Enthalpy ofFusion)

It is preferred that the polylactic acid resin composition of thepresent invention have a peak temperature of its melting point withinthe range of from 210 to 240° C. in measurement using a scanningdifferential calorimeter. That it is lower than 210° C. is undesirablefor the object of the present invention, the object of which is toimprove heat resistance. It is more preferably 215° C. or higher,particularly preferably 220° C. or higher, and most preferably 225° C.or higher. Causes of the occurrence of the case that the peaktemperature of the melting point is lower than 210° C. include thermaldegradation of the polylactic acid resin and melting point loweringcaused by transesterification, and it is preferred to minimize thethermal hysteresis during the process. In addition, it is preferred thatthe enthalpy of fusion of an absorption with a peak at 210 to 240° C.observed at a temperature raising rate of 20° C. per minute afterrapidly cooling immediately after raising the temperature to 280° C. andmaintaining the temperature for 3 minutes be 60 J/g or more. That theenthalpy of fusion is less than 60 J/g is undesirable for the presentinvention whose object is to improve heat resistance because thestereocomplex formation efficiency is low. The enthalpy of fusion ispreferably 65 J/g or more and more preferably 70 J/g or more. Moreover,it is preferred that also an absorption observed at a temperatureraising rate of 20° C. per minute after rapidly cooling immediatelyafter raising the temperature to 280° C. and maintaining the temperaturefor 3 minutes have a peak at 215° C. or higher, particularly at 220° C.or higher. The melting point may be 250° C. at most; an effect in heatresistance will be only saturated even if the melting temperatureexceeds this value. The enthalpy of fusion may be 80 J/g at most; aneffect in stereocomplex crystal formation efficiency will be onlysaturated even if the enthalpy of fusion exceeds this value. Also whenproducing a drawn film, raising its temperature to 280° C. and thenrapidly cooling it, and measuring the amount of enthalpy of fusion, thebelow described method for measuring “the amount of enthalpy of fusion(ΔHm) after heating to 280° C. and then cooling rapidly” can be carriedout and data can be collated.

The information about the PLLA and the PDLA to be used is as describedabove.

(Sheet)

The polylactic acid resin composition of the present invention is usedsuitably for a sheet. As to processing into a sheet, the sheet can beobtained by subjecting sheets of PLLA and sheets of PDLA to multilayerstacking and rolling by the above-described methods, and it is alsopossible to directly convert a material multilayered by using a staticmixer or a multilayer feed block into a sheet by using a T-shaped die orthe like. Moreover, it is preferred in the present invention that it ispossible to remelt pellets prepared by repelletizing a multilayeredstrand-shaped or sheet-shaped resin and then form a sheet. Although thecasting method to be used in the case of directly forming a sheet is thesame as ordinary methods for forming a sheet, it is preferred todirectly extrude a material melt-stacked as described above and thenform a sheet by bringing the material into contact firmly with a chillroll adjusted to 20 to 60° C. Although the lower the chill rolltemperature, the more preferred, the chill roll temperature ispreferably within the range of from 30 to 60° C. in order to suppressdevelopment of pollution with time due to deposition of an oligomer.Since the crystal size is prevented from becoming large due tomultilayering in the present invention even in casting by slow cooling,that only a small influence is given on the fall of drawability is oneof the features of the present invention. The thickness of the sheet ispreferably within the range of from 10 to 1000 μm.

The aforementioned thermally treated melt is cast with a T-shaped die orthe like. The die temperature is within the range of from 220 to 300°C., preferably from 230 to 290° C., more preferably from 240 to 280° C.If the temperature becomes excessively low, the melt viscosity willbecome excessively high and, therefore, roughness on a surface or thelike will occur, so that the appearance will deteriorate. That thetemperature becomes excessively high is undesirable because if so,thermal decomposition or fall of melting point of the polylactic acidresin occurs. As to causes of rupture that occurs in drawing whenobtaining a drawn film from a cast sheet, that crystals of astereocomplex formed when stacking layers do not melt completely duringheat treatment and some of them remain is conceived to be one of thecauses. In this case, it is preferred to raise the temperature of theheat treatment. Also in producing a molded article other than a film, aproblem may arise due to occurrence of sinking or the like if crystalsremain during melting. Also in such a case, it is preferred to raise thetemperature of the heat treatment.

The haze of a sheet obtained from the polylactic acid resin of thepresent invention is preferably within the range of from 0.1 to 50%.That it exceeds 50% is undesirable because if so, applications arerestricted due to low transparency.

(Drawn Film)

The polylactic acid resin composition of the present invention, whichcan be used also in an undrawn state without being drawn as describedabove, is preferably drawn from the viewpoint of mechanical propertiesand it is used suitably for drawn film applications. As a drawingmethod, a method by blown film production can be used as well assequential biaxial drawing or simultaneous biaxial drawing of theabove-described extruded sheet.

As to processing into a drawn film, it can be obtained by bringing amaterial into contact firmly with a chill roll having a surfacetemperature of 10 to 50° C. by electrostatic contact system or touchroll system by melt-extrusion using the method disclosed in theforegoing section of sheet formation, thereby cooling it rapidly, andthen biaxially drawing it.

In the case of subsequent biaxial drawing, the first stage drawingtemperature preferred in the present invention is within the range offrom 70 to 120° C., and more preferably within the range of from 75 to110° C. If it is lower than the range provided above, it is difficult toperform drawing, whereas if it exceeds the range, it becomes difficultto perform drawing due to occurrence of crystallization. The first stagedrawing may be carried out either at one time or in several times.

The second stage drawing means drawing in a direction perpendicular tothe first stage drawing. In the second stage drawing, while the drawingratio is preferably 3 to 8 times, it is determined in view of balancewith mechanical properties in balance with the first stage drawing andit is not particularly limited.

In the case of simultaneous biaxial drawing, the drawing temperaturepreferred in the present invention is within the range of from 70 to120° C. If it is lower than 70° C., it is difficult to perform drawing,whereas that it exceeds 120° C. is undesirable because if so,crystallization occurs. The drawing ratio is preferably within the rangeof 2 to 8 times, expressed in area.

Heat setting is performed after drawing and it is preferably carried outwithin the range of from 140 to 220° C. If the temperature is lower than140° C., the thermal crystallization efficiency is low, whereas that thetemperature exceeds 220° C. is undesirable because if so, a film willmelt, resulting in high unevenness in thickness.

Relaxation is carried out after heat setting, and the relaxation ratiois preferably within the range of from 0.5 to 10%. The heat shrinkage ofa resulting drawn film is preferably within the range of from 0 to 5%after a heat treatment of 150° C.×30 minutes. That it exceeds 5% isundesirable for the present invention which intends to improve heatresistance, and that is also undesirable because dimensional error iscaused in various processing steps such as printing and the occurrenceof wrinkle or the like along with shrinkage is caused.

The thickness of a resulting drawn film, which is not particularlylimited, is from 3 to 300 μm, preferably from 4 to 250, and morepreferably from 5 to 200 μm. A film of 3 μm or less in thickness isundesirable for use in intended applications due to insufficientstiffness or strength of the film itself. That the film is thicker than200 μm is undesirable because of poor productivity. The thickness of afilm is determined from the thickness of an extruded film and thedrawing ratio of the following drawing, and it is preferred to set theaforementioned conditions in view of the final thickness.

Although the transparency of the film of the present invention can bedetermined by haze measurement or the like, the haze is preferablywithin the range of from 0.05 to 20% when transparency is needed or thehaze is preferably 80% or more when hiding ability is required. The hazevalue, which depends on a haze value inherent to the resin constitutingthe film, the types of additives and their amounts added, is usuallydetermined according to the amount of a lubricant for imparting slippingability to the film, and that the haze is less than 0.05% is undesirablebecause if so, the amount of the lubricant is so small that the slippingability of the film is insufficient. That the haze is 20% or more isundesirable because if so, contents are difficult to be seen when thefilm is used as a wrapping material. In use for applications likesynthetic paper, that the haze is lower than 80% is undesirable becauseif so, contents are undesirably seen well due to low hiding ability.

The unevenness in thickness of a resulting drawn film is preferablywithin the range of from 0 to 10% in the longitudinal direction and thetransverse direction. That it exceeds 10% is undesirable because if so,the appearance of a wound-up roll is poor or unevenness occurs inprinting. In order to reduce unevenness in thickness, it is preferred toadjust the drawing stress during drawing or tension at the time of heattreatment and, concretely, it is attained by appropriately adjusting thetemperature or the drawing rate at the time of drawing and thetemperature at the time of heat setting.

Although the strength of the film of the present invention variesdepending upon measurement conditions and is represented by a valuemeasured at an original sample length of 40 mm and a speed of acrosshead portion of 200 mm/min by using a tensile tester, the elasticmodulus is preferably at least 2 GPa. That it is less than 2 GPa isundesirable because if so, the film becomes easy to stretch duringvarious types of processing such as printing. Moreover, the upper limitis preferably 5 GPa. That it exceeds 5 GPa is undesirable in respect ofproductivity.

The elongation at break is preferably 10% or more, and more preferably20% or more. If the elongation at break is less than 10%, productivitymay fall due to frequent occurrence of rupture during film formation orvarious types of processing or difficulty may arise in handling. Theupper limit is preferably 300%, and more preferably 150% or less. Thatit exceeds 300% is conversely undesirable because if so, the filmbecomes easy to stretch.

Such a film high in strength and high in degree of elongation can not beobtained until a stereocomplex is formed sufficiently and is drawnsufficiently by the methods described previously. The strength anddegree of elongation within the aforementioned ranges can be adjusted bydrawing conditions such as drawing ratio, introduction of a small amountof copolymerization to the polylactic acid (1) and the polylactic acid(d), and so on.

As to the slipping ability of the film of the present invention, it ispreferred that both a coefficient of dynamic friction and a coefficientof static friction be from 0.2 to 1.0. That the coefficients are lessthan 0.2 is undesirable because if so, the film is very easy to slipand, therefore, winding slide of a film roll or the like easily occurs.That the coefficients exceed 1.0 is undesirable because if so, itbecomes difficult to wind up the film into a film roll and generation ofwrinkles or the like caused by poor slipping ability become noticeable.In order to impart such slipping ability, besides the addition of alubricant to a resin, there can be used methods such as laminating alubricant-containing layer on at least one side as a surface layer orforming a coat layer containing a lubricant on a surface. Particularly,a method of laminating a layer containing a lubricant as only a surfacelayer is the most preferable method in respect of compatibility oftransparency with slipping ability, and especially laminating of a coatlayer by an in-line coating method is most preferred. In this case, acast sheet is uniaxially drawn and then is applied to a substrate filmbefore crystal orientation is completed. Examples of the applying methodinclude a reverse roll coating method, a gravure coating method, a kisscoating method, a roll brushing method, a spray coating method, an airknife coating method, a wirebar bar coating method, a pipe doctormethod, an immersion coating method, and a curtain coating method, andthese methods can be conducted alone or in combination.

The solid concentration of the coating liquid is usually 30% by mass orless, and preferably is 10% by mass or less. That the concentration is30% by mass or more is undesirable because if so, the viscosity is highand it is difficult to do uniform application. The applied amount of thecoating liquid, expressed in solid content, is from 0.005 to 5 g/m² andpreferably is from 0.02 to 0.5 g/m². That the applied amount becomes0.005 g/m² or less is undesirable because if so, a lubricant comes offor sufficient adhesion strength to an adhesion improving layer is notobtained. If it becomes 5 g/m² or more, blocking occurs and, therefore,there is a problem in practical use.

The film on which the coating liquid is applied is introduced to atenter for drawing and heat setting and is heated to form a stablecoating, so that a polyester-based laminated film is formed. The degreeof cleanness at the time of applying a coating liquid is preferablyclass 1000 or less in order to reduce adhesion of dust.

The film of the present invention is subjected to various types ofprocessing, examples of which include printing, lamination of a sealant,and sealing. In many cases in printing or sealant lamination, a filmreceives tension in its longitudinal direction and heat, so that it isbrought into a condition where elongation or wrinkles are easilyproduced. Since local elongation and wrinkles or the like caused by thelocal elongation will occur when the slipping ability of a film is poor,it is preferred that the slipping ability of a film be sufficientlyhigh, and it is preferred that a coefficient of static friction and acoefficient of dynamic friction be 0.4 or less. Since heat and tensionare applied at the time of processing and, as a result, a film isundesirably stretched, the elastic modulus is preferably within therange of from 2 to 5 GPa. If it is less than 2 GPa, the film becomeseasy to stretch, whereas productivity decreases in the productionperformed under a condition exceeding 5 GPa. Moreover, since a high heatshrinkage will cause generation of wrinkles, the heat shrinkage at 120°C. for 5 minutes is preferably 10% or less.

In order to make the film of the present invention be superior inprintability, besides treatment such as corona treatment, an easilyprintable layer can further be laminated by a method such as in-linecoating or off-line coating. The coated amount is preferably within therange of from 0.005 to 5 g/m². Since the film of the present inventionis high in resistance to solvents because of its constitution, varioussolvents can be used, but application conditions should be determined inconsideration of the kind of solvent, temperature, and tension in orderto minimize the influence on various properties due to the heat and thetension to be applied to the film. The concrete solvent includessolvents having a boiling point of 200° C. or lower, and concretely,water, alcohols, ketones, esters, aromatics, hydrocarbons,chlorine-containing solvents, and so on can be used; the dryingtemperature is 200° C. or lower, and preferably is 180° C. or lower.

The film of the present invention is superior in slipping abilitybecause it essentially has a layer in which a lubricant is added as theoutermost layer. Concretely, the coefficient of static friction and thecoefficient of dynamic friction are 0.4 or less, and because of the factthat the film is superior in slipping ability, neither wrinkles norlocal unevenness is found in extrusion lamination processing orprinting.

The drawn film of the present invention has a high melting point despitethe fact that it is constituted of a polylactic acid resin. Therefore,it excels in heat resistance from being low heat shrinkage.

From the polylactic acid of the present invention, not only films butalso various types of molded articles can be produced by injectionmolding, extrusion forming, blow molding, and so on. At this time, evenif the polylactic acid is melted at 260 to 300° C., thereby fullymelting stereocomplex crystals, the resulting molded article is high inefficiency to reform stereocomplex crystals. Therefore, it is possibleto form drawn bottles or fibers, and the resulting molded article canhave superior properties such as mechanical properties, solventresistance and gas barrier property, which stereocomplexes have.

EXAMPLES

The present invention is described more concretely with reference toexamples, but the invention is not limited these examples. Theproperties shown in the examples are those measured or evaluated by thefollowing methods.

1. Weight Average Molecular Weight

8 mg of a sample was dissolved in 4 ml of chloroform to prepare a samplesolution. Then, the solution was filtered through a 0.2-μm membranefilter, followed by GPC measurement of the resulting sample solution.

Instrument: TOSOH HLC-8220GPC

Column: TSK gel Super Multipore HZ-M X2+TSK gel SuperHZ2000 (TOSOH)

Solvent: chloroform

Flow rate: 0.35 ml/min

Temperature: 40° C.

Detector: RI

A molecular weight was calculated in terms of standard polystyrenes.

2. Amount of Remaining Lactide (Wt %)

A sample was dissolved in chloroform D, and the amount of remaininglactide was calculated from the ratio of the integral value of theprotons originating in a polylactic acid to the integral value of theprotons originating in the remaining lactide obtained by using a 400 MHznuclear magnetic resonance spectrometer (NMR).

3. Glass Transition Temperature Tg (° C.)

Measurement was conducted in accordance with JIS K7122. By using adifferential scanning calorimeter, model DSC-60 manufactured by ShimadzuCorporation, about 10.0 mg of a sample was prepared and measured withinthe range of 30° C. to 280° C. at a temperature raising rate of 20°C./min, so that a DSC curve was obtained. The temperature of the pointof inflection due to glass transition was read from it.

4. Melting Point Tm (° C.)

Measurement was conducted in accordance with JIS K7122. By using adifferential scanning calorimeter, model DSC-60 manufactured by ShimadzuCorporation, about 10.0 mg of a sample was prepared and measured withinthe range of 30° C. to 280° C. at a temperature raising rate of 20°C./min, so that a DSC curve was obtained. The melting peak temperaturewas read from it.

5. Enthalpy of Fusion (ΔHm) for No Application of Treatment at 280° C.

Measurement was conducted in accordance with JIS K7122. By using adifferential scanning calorimeter, model DSC-60 manufactured by ShimadzuCorporation, about 10.0 mg of a sample was prepared and measured withinthe range of 30° C. to 280° C. at a temperature raising rate of 20°C./min, so that a DSC curve was obtained. The enthalpy of fusion of astereocomplex was determined from the resulting curve.

6. Enthalpy of Fusion (ΔHm) after Raising Temperature to 280° C. andRapidly Cooling

Measurement was conducted in accordance with JIS K7122. By using adifferential scanning calorimeter, model DSC-60 manufactured by ShimadzuCorporation, about 10.0 mg of a sample was prepared and its temperaturewas raised to 280° C. and then maintained for 3 minutes. Then, thesample was taken out and it was immersed in liquid nitrogen for 5minutes. Subsequently, the sample was picked out from the liquidnitrogen and it was left at rest at room temperature for 10 minutes.Then, it was measured within the range of from 30 to 330° C. at atemperature raising rate of 20° C./min, and from the resulting curve wasdetermined the enthalpy of fusion of a stereocomplex.

7. Mechanical Properties (Elastic Modulus, Strength at Break)

Measurement was conducted in accordance with JIS K7113. A sample 10 mmin width and 100 mm in length along the longitudinal direction and thetransverse direction of a film, respectively, was cut out with a razor,and the cut piece was used as a sample. Measurement was performed underconditions including a distance between chucks of 40 mm and a drawingrate of 200 mm/min, and the average of five measurements was used. As ameasuring instrument, an Autograph AG5000A manufactured by ShimadzuCorporation was used.

8. Dimensional Change (Degree of Heat Shrinkage)

The test was performed in accordance with the method for testingdimensional change provided in JIS C2318 except for adjusting thetesting temperature to 150° C. and the heating time to 30 minutes.

9. Transparency Test

A haze value was measured by using a haze meter manufactured by ToyoSeiki Seisaku-sho, Ltd. in accordance with JIS K6714.

Production Example 1 Production of PLLA

A 2-L reaction vessel made of SUS304 equipped with a stirrer, athermometer, and a nitrogen inlet was charged with 5000 g of L-lactide,and the lactide was melted at a temperature of 120° C. under stirringunder a nitrogen atmosphere. Then, 1.4 g of tin octylate, 5.0 g ofethylene glycol as an initiator, and 2.9 g of trimethyl phosphate wereadded. The weight average molecular weight of the lactide at the time ofits addition was 500 or less. Then, the temperature was raised to 180°C. and polymerization was carried out for 1.5 hours. Then, the pressurewas held at a reduced pressure of 0.1 Torr for 0.5 hours to prepare apolylactic acid (PLLA-1). The weight average molecular weight of theresulting resin was 171,000 and the amount of remaining lactide was1.1%.

Production Example 2 Production of PDLA

A polylactic acid resin (PDLA-1) was obtained by carrying outpolymerization similarly except for using D-lactide instead of L-lacticacid. The weight average molecular weight of the resulting resin was163,000 and the amount of remaining lactide was 1.3%.

Example 1

Using a melt pressing machine set at 200° C., PLLA-1 was sandwichedbetween Teflon (registered trademark) sheets, pressed and held for 3minutes, and then rapidly cooled with cold water, so that a sheet with athickness of 100 μm was obtained. A sheet of PDLA-2 was also produced ina similar way. These sheets including five of each type were stackedalternately and pressed at 200° C., so that a 200-μm thick sheet having10 layers was obtained. In the same method, ten-layer sheets, ten sheetsin total, were produced and these sheets were stacked so that PLLA-1would contact with PDLA-1, and then they were pressed at 200° C., sothat a 100-μm thick sheet having 100 layers was obtained. In the samemethod, ten-layer sheets, ten sheets in total, were produced and thesesheets were stacked in a similar manner so that PLLA-1 would contactwith PDLA-1, and then they were pressed at 230° C., so that a 100-μmthick sheet having a theoretical number of layers of 100 was obtained.In the same method, 100-layer sheets, ten sheets in total, were producedand these sheets were stacked in a similar manner so that PLLA-1 wouldcontact with PDLA-1, and then they were pressed at 280° C., so that a100-μm thick sheet having a theoretical number of layers of 1000 wasobtained. The thickness of each layer was calculated to be 0.1 μm. Thissheet was pressed and then rapidly cooled with cold water, so that anamorphous sheet was obtained. When this sheet was measured by DSC with atemperature raising rate of 20° C./min, the Tg was 52° C. and nocrystallization peaks of the PLLA and the PDLA were observed. The PLLAand the PDLA had a melting point of 167° C. and a ΔHm of 4.1 J/g. Thestereocomplex PLA had a Tm of 233° C. and a ΔHm of 74.5 J/g. When thiswas heated to 280° C., then cooled rapidly with liquid nitrogen, andmeasured at a temperature raising rate of 20° C./min again, the PLLA andthe PDLA had a Tm of 166° C. and a ΔHm of 16.6 J/g. The stereocomplexPLA had a Tm of 223° C. and a ΔHm of 69 J/g. Hence, a stereocomplex wasformed efficiently in the elevation of temperature during the DSCmeasurement.

Comparative Example 1

Using a melt pressing machine set at 200° C., PLLA-1 was sandwichedbetween Teflon (registered trademark) sheets, pressed and held for 3minutes, and then rapidly cooled with cold water, so that a sheet with athickness of 100 μm was obtained. A sheet of PDLA-1 was also produced ina similar way. These sheets including five of each type were stackedalternately and pressed at 280° C., so that a 100-μm thick sheet having10 layers was obtained. The thickness of each layer was calculated to be10 μm. When this sheet was measured by DSC with a temperature raisingrate of 20° C./min, the Tg was 52° C. and the crystallization peaks ofthe PLLA and the PDLA were 103° C. and 123° C., respectively. The PLLAand the PDLA had a melting point of 167° C. and a ΔHm of 23.3 J/g. Thestereocomplex PLA had a Tm of 221° C. and a ΔHm of 25.1 J/g. When thiswas heated to 280° C., then cooled rapidly with liquid nitrogen, andmeasured at a temperature raising rate of 20° C./min again, the PLLA andthe PDLA had a Tm of 170° C. and a ΔHm of 23.9 J/g. The stereocomplexPLA had a Tm of 222° C. and a ΔHm of 34.1 J/g. The efficiency ofstereocomplex formation was low.

Examples 2 to 5, Comparative Examples 2 and 3

Sheets were prepared in the same manner as that used in Example 1,except for partly changing conditions. The results are shown in Table 1.

Example 6

The PLLA resin described in Production Example 1 and the PDLA describedin Production Example 2 were vacuum dried separately at 100° C.overnight, and their pellets were fed to two extruders. The pellets weremelted at 220° C. and PLLA and PDLA were stacked by using a 16-elementstatic mixer with a temperature gradient of 200/240/280° C. from theinlet toward the outlet of the static mixer. Then, a strand was cooledwith water to solidify and cut to afford pellets. Physical properties ofthe resulting resin composition are shown in Table 1.

Comparative Example 4

The PLLA resin described in Production Example 1 and the PDLA describedin Production Example 2 were vacuum dried separately at 100° C.overnight, and pellets of the respective resins were blended in a weightratio of 50/50 and fed into a single screw extruder having barreltemperatures of 150/200/280° C. and L/D=30 (dimension ratio). Meltingand mixing were conducted in the extruder, and then a strand was cooledwith water to solidify and cut to afford pellets. Physical properties ofthe resulting resin composition are shown in Table 1.

Example 7

The rapidly cooled sheet with a theoretical number of layers of 1000obtained in Example 1 was cut into a length of 40 mm and a width of 50mm, fixed to a hand-rotated drawer with a distance between chucks of 20mm. Then, it was preliminarily heated in a hot air oven of 85° C. for 3minutes and uniaxially drawn to 2.5 times at a deformation rate of500%/min. This was then drawn at the same temperature, the same rate,and the same ratio to a direction of 90° with respect to that of theuniaxial drawing. The biaxially drawn sample was heat set in an oven of100° C. while being fixed, so that a biaxially drawn film was obtained.The resulting film was transparent. In dynamic viscoelasticitymeasurement, lowering of elastic modulus was not observed in a region offrom 80 to 180° C. and the elastic modulus lowering temperature was 190°C.; hence, heat resistance was improved greatly in comparison tobiaxially drawn films of only ordinary PLLA or PDLA.

Comparative Example 5

The rapidly cooled sheet with a theoretical number of layers of 100obtained in Comparative Example 1 was cut into a length of 40 mm and awidth of 50 mm, fixed to a hand-rotated drawer with a distance betweenchucks of 20 mm. Then, it was preliminarily heated in a hot air oven of85° C. for 3 minutes and uniaxially drawn to 2.5 times at a deformationrate of 500%/min. This was then drawn at the same temperature, the samerate, and the same ratio to a direction of 90° with respect to that ofthe uniaxial drawing. The biaxially drawn sample was heat set in an ovenof 180° C. while being fixed, so that a biaxially drawn film wasobtained. The resulting film was transparent. In dynamic viscoelasticitymeasurement, great fall of elastic modulus near 160° C. was observed andit had only heat resistance equivalent to biaxially drawn films of onlyordinary PLLA or PDLA.

Example 8

Using a melt pressing machine set at 200° C., PLLA-1 was sandwichedbetween Teflon (registered trademark) sheets, pressed and held for 3minutes, and then rapidly cooled with cold water, so that a sheet with athickness of 100 μm was obtained. A sheet of PDLA-2 was also produced ina similar way. These sheets including five of each type were stackedalternately and pressed at 200° C., so that a 200-μm thick sheet having10 layers was obtained. In the same method, ten-layer sheets, ten sheetsin total, were produced and these sheets were stacked so that PLLA-1would contact with PDLA-1, and then they were pressed at 200° C., sothat a 100-μm thick sheet having 100 layers was obtained. In the samemethod, ten-layer sheets, ten sheets in total, were produced and thesesheets were stacked in a similar manner so that PLLA-1 would contactwith PDLA-1, and then they were pressed at 230° C., so that a 100-μmthick sheet having a theoretical number of layers of 100 was obtained.In the same method, 100-layer sheets, ten sheets in total, were producedand these sheets were stacked in a similar manner so that PLLA-1 wouldcontact with PDLA-1, and then they were pressed at 250° C., so that a100-μm thick sheet having a theoretical number of layers of 1000 wasobtained. The thickness of each layer was calculated to be 0.1 μm. LikeExample 1, this satisfies the present invention as a sheet-formedcomposition. This sheet was pressed and then cooled rapidly with coldwater. This sheet was cut to have a length of 40 mm and a width of 50mm, fixed to a hand-rotated drawer with a distance between chucks of 20mm. Then, it was preliminarily heated in a hot air oven of 85° C. for 3minutes and uniaxially drawn to 2.5 times at a deformation rate of500%/min. However, rupture was observed at a draw ratio of 1.5 times ormore, resulting in failure in drawing. This result explains that it ismeant that the resin compositions of the inventions described as thefirst and the second provided above are heated to a temperature of 280°C. or higher before conducting processing such as converting into adrawn film, thereby once melting stereocomplex crystals (the inventiondescribed in the third).

Example 9

The PLLA resin described in Production Example 1 and the PDLA describedin Production Example 2 were vacuum dried separately at 100° C.overnight, and their pellets were fed to two extruders. The pellets weremelted at 220° C. and PLLA and PDLA were stacked by using an 18-elementstatic mixer with a temperature gradient of 200/240/280° C. from theinlet toward the outlet of the static mixer, extruded through a T-shapeddie heated to 280° C., and cooled to solidify into a sheet form by usinga cooling roll adjusted to 20° C., so that a multilayer undrawn sheetwas produced. The ratio of the discharge amounts of the two extruderswas adjusted to 1:1. The thickness of the undrawn sheet was 200 μm. Thissheet had a Tg of 55° C. and a melting point of 232° C. This sheet wassubjected to a preheating treatment at a temperature of 75° C. first andthen subjected to MD drawing to 3.5 times at a drawing temperature of85° C. and a distortion rate of 5000%/min. This sheet was subsequentlyintroduced to a tenter continuously, then subjected to TD drawing to 3.5times in a post-heating zone of 85° C. and a drawing zone of 85° C.,further subjected to thermal fixation at 180° C. and transverserelaxation in 3%, then cooled, and further subjected to trimming of bothedge portions, so that a 15-nm thick biaxially drawn stereocomplex PLAresin film was obtained. The film properties at this time are shown inTable 1.

Comparative Example 6

The PLLA resin described in Production Example 1 and the PDLA describedin Production Example 2 were vacuum dried separately at 100° C.overnight, and the two kinds of pellets were blended in a weight ratioof 50/50, kneaded at 220° C. in an extruder, extruded through a T-shapeddie of 220° C., and cooled to solidify into a sheet form by using acooling roll adjusted to 20° C., so that an undrawn sheet was produced.The thickness of the undrawn sheet was 200 nm. By DSC measurement, itwas found that this sheet had a Tg of 55° C. and a melting point on ahigher temperature side of 216° C. After the measurement, the sample washeld at 280° C. for 5 minutes and then measured again under rapidcooling; the melting point on the higher temperature side was 189° C.This result reproduces the result of non-patent document 1 cited in thisdescription, and it was found that the presently inventeD-formation ofmultiple layers is an essential factor for the purpose of maintainingthe melting point of a stereocomplex crystal. The above-described sheetwas subjected to a preheating treatment at a temperature of 75° C. andthen subjected to MD drawing to 3.5 times at a drawing temperature of85° C. and a distortion rate of 5000%/min. This sheet was subsequentlyintroduced to a tenter continuously and then subjected to TD drawing to3.5 times in a post-heating zone of 85° C. and a drawing zone of 85° C.The film was melted by thermal fixation at 180° C. and a film superiorin heat resistance could not be obtained.

Example 10

The PLLA resin described in Production Example 1 and the PDLA describedin Production Example 2 were vacuum dried separately at 100° C.overnight, and their pellets to which 3% by weight of a polyamideelastomer resin (Pebax4033 produced by ARKEMA) had been added were fedto two extruders. The pellets were melted at 220° C. and PLLA and PDLAwere stacked by using a 12-element static mixer with a temperaturegradient of 200/240/280° C. from the inlet toward the outlet of thestatic mixer, extruded through a T-shaped die heated to 280° C., andcooled to solidify into a sheet form by using a cooling roll adjusted to20° C., so that a multilayer undrawn sheet was produced. The ratio ofthe discharge amounts of the two extruders was adjusted to 1:1. Thethickness of the undrawn sheet was 250 μm. By the addition of thepolyamide elastomer resin, the pollution of the chilled roll inmelt-casting was greatly suppressed. This sheet had a Tg of 55° C. and amelting point of 222° C. This sheet was subjected to a preheatingtreatment at a temperature of 75° C. first and then subjected to MDdrawing to 4 times at a drawing temperature of 85° C. and a distortionrate of 7000%/min. This sheet was subsequently introduced to a tentercontinuously, then subjected to TD drawing to 4 times in a post-heatingzone of 85° C. and a drawing zone of 90° C., further subjected tothermal fixation at 180° C. and transverse relaxation in 3%, thencooled, and further subjected to trimming of both edge portions, so thata 15-μm thick biaxially drawn stereocomplex PLA resin film was obtained.The film properties at this time are shown in Table 1. The resultingfilm was superior also in pinhole resistance.

Example 11

A commercially available PLLA (Lacea H100 produced by Mitsui Chemicals,Inc.) and the PDLA described in Production Example 2 were vacuum driedseparately at 100° C. overnight, and their pellets were fed to twoextruders. After melting them at 220° C., the PLLA and the PDLA werestacked by using a 4-element static mixer of 220° C. This was made passthrough a multilayer feed block having 256 layers with a temperaturegradient of from 250 to 280° C. from the inlet toward the out let,thereby being stacked into about 4000 layers, and this was extrudedthrough a T-shaped die heated to 280° C., and cooled to solidify into asheet form by using a cooling roll adjusted to 35° C., so that amultilayer undrawn sheet was produced. The ratio of the dischargeamounts of the two extruders was adjusted to 1:1. The thickness of theundrawn sheet was 400 μm. This sheet had a Tg of 55° C. and a meltingpoint of 229° C. This sheet was subjected to a preheating treatment at atemperature of 80° C. first and then subjected to MD drawing to 3.5times at a drawing temperature of 85° C. and a distortion rate of7000%/min. This sheet was subsequently introduced to a tentercontinuously, then subjected to TD drawing to 3.5 times in apost-heating zone of 85° C. and a drawing zone of 90° C., furthersubjected to thermal fixation at 180° C. and transverse relaxation in3%, then cooled, and further subjected to trimming of both edgeportions, so that a 30-μm thick biaxially drawn stereocomplex PLA resinfilm was obtained. The film properties are shown in Table 1.

Example 12

The PLLA resin described in Production Example 1 and the PDLA describedin Production Example 2 were vacuum dried separately at 100° C.overnight, and fed to two extruders. An item prepared by melting at 220°C. and stacking PLLA and PDLA by using a 10-element static mixer with atemperature gradient of 200/240/280° C. from the inlet toward the outletof the static mixer was placed on both skin layer sides of a three-layerfeed block. A resin prepared by mixing the PLLA and an aliphaticaromatic polyester resin (ECOFLEX, produced by BASF) in a weight ratioof 50/50 was used as a resin of the core layer side, so that a two-kind,three-layer constitution of A/B/A type (thickness ratio: 25/50/25) wasproduced. This was extruded through a T-shaped die heated to 280° C.,and cooled to solidify into a sheet form by using a cooling rolladjusted to 20° C., so that a multilayer undrawn sheet was produced. Theratio of the discharge amounts of the PLLA and the PDLA was adjusted to1:1. The thickness of the undrawn sheet was 250 μm, wherein the skinlayers were each 50 μm thick. The Tg and the melting point of the skinlayer sides of this sheet were 55° C. and 224° C., respectively. Thissheet was subjected to a preheating treatment at a temperature of 75° C.first and then subjected to MD drawing to 4 times at a drawingtemperature of 85° C. and a distortion rate of 7000%/min. This sheet wassubsequently introduced to a tenter continuously, then subjected to TDdrawing to 4 times in a post-heating zone of 85° C. and a drawing zoneof 90° C., further subjected to thermal fixation at 180° C. andtransverse relaxation in 3%, then cooled, and further subjected totrimming of both edge portions, so that a 15-μm thick biaxially drawnstereocomplex PLA resin film was obtained. The film properties at thistime are shown in Table 1. By introducing the above-mentioned layer intoa core layer, pinhole resistance was improved greatly.

TABLE 1 Com- Com- parative Exam- Exam- Exam- Exam- Comparative parativeExam- Comparative Example 1 Example 1 ple 2 ple 3 ple 4 ple 5 Example 2Example 3 ple 6 Example 4 L-form content in PLLA [%] 99.5 99.5 99.5 99.599.5 99.5 99.5 99.5 99.5 99.5 D-form content in PDLA [%] 99.5 99.5 99.599.5 99.5 99.5 99.5 99.5 99.5 99.5 Other component in PLLA — — — — — — —— — — layer (L layer) Other component in PDLA — — — — — — — — — — layer(D layer) Other layer Stacking state of L layer and Stacked StackedStacked Stacked Stacked Stacked Stacked Stacked Stacked Mixed D layeralternately alternately alter- alter- alter- alter- alter- alter- alter-nately nately nately nately nately nately nately Number of total layers1000 10 5000 200 1000 1000 1000 10000 ≈1000 — Thickness of Llayer:thickness 50:50 50:50 50:50 50:50 42:58 65:35 20:80 50:50 50:50Mixture ratio of D layer 50:50 Thickness of one L layer [μm] 0.1 10 0.020.5 0.08 0.13 0.04 0.001 ≈0.1 — Thickness of one D layer [μm] 0.1 100.02 0.5 0.12 0.07 0.16 0.001 ≈0.1 — Total thickness of the sheet [μm]100 100 100 100 100 100 100 100 100 100 Heating and rolling method ◯ ◯ ◯◯ ◯ ◯ ◯ ◯ Temperature of rolling [° C.] 200, 200, 200, 200, 200, 200,200, 200, 200, 280, 200, 200, 200, 200, 200, 200, 230, 230, 280 230,230, 230, 230, 280 250, 265 270 280 280, 280 280, 280 Static mixermethod ◯ Number of elements [—] 16 Temperature gradient [° C.] 220, 200,240, 280 Feed block method Structure Temperature gradient [° C.] Meltingand mixing ◯ Temperature of mixing [° C.] 280 Melting point [° C.] 233221 229 230 233 233 222 208 232 225 Enthalpy of fusion after [J/g] 6934.1 70.4 61.4 65.3 60.8 28.6 34.1 68.6 12.1 raising 280° C. and rapidlycooling Temperature of extruding [° C.] Biaxially drawablity Heatresistance of the film Haze value of the film [%] 2.4 10.3 2.3 4.3 2.93.3 1.9 1.2 2.2 1.1 Comparative Comparative Example7 Example 5 Example 8Example 9 Example 6 Example 10 Example 11 Example12 L-form content inPLLA [%] 99.5 99.5 99.5 99.5 99.5 99.5 99.3 99.5 D-form content in PDLA[%] 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 Other component in PLLA — —— — — Pebax4033 — — layer (L layer) 3 wt % Other component in PDLA — — —— — Pebax4033 — — layer (D layer) 3 wt % Other layer Core layer PLLA +Ecoflex (50/50) Stacking state of L layer and Stacked Stacked StackedStacked Mixed Stacked Stacked Stacked D layer alternately alternatelyalternately alternately alternately alternately alternately Number oftotal layers 1000 10 1000 ≈1000 — ≈1000 4000 ≈1000 Thickness of Llayer:thickness 50:50 50:50 50:50 50:50 Mixture ratio 50:50 50:50 50:50of D layer 50:50 Thickness of one L layer [μm] 0.1 10 0.1 ≈0.1 — ≈0.10.1 ≈0.1 Thickness of one D layer [μm] 0.1 10 0.1 ≈0.1 — ≈0.1 0.1 ≈0.1Total thickness of the sheet [μm] 100 100 100 200 200 250 400 250Heating and rolling method ◯ ◯ ◯ Temperature of rolling [° C.] 200, 200,200, 200, 280 200, 230, 230, 280 250 Static mixer method ◯ ◯ ◯ ◯ Numberof elements [—] 18 12 4 10 Temperature gradient [° C.] 220, 220, 220220, 200, 200, 240, 240, 240, 280 280 280 Feed block method ◯ Structure4 division 4 times Temperature gradient [° C.] 250, 280 Melting andmixing ◯ Temperature of mixing [° C.] 220 Melting point [° C.] 233 221233 230 216 228 229 224 Enthalpy of fusion after [J/g] 69 34.1 69 69.812.1 60.4 64.9 62.1 raising 280° C. and rapidly cooling Temperature ofextruding [° C.] 280 220 280 280 280 Biaxially drawablity ◯ Δ X ◯ Δ ◯ ◯◯ Heat resistance of the film ◯ X — ◯ X ◯ ◯ ◯ Haze value of the film [%]0.8 14.3 — 1.4 18.2 3.1 1.1 2.9

INDUSTRIAL APPLICABILITY

The polylactic acid resin disclosed in the present invention and thepolylactic acid film obtained therefrom are capable of being used athigher temperatures in comparison with a usual polylactic acid resin andpolylactic acid resin film, superior in various processabilities such asprintability and in appearance such as transparency, high inproductivity, and high in industrial usefulness. They can be usedsuitably as industrial materials such as molding materials as well asmaterials for wrapping foods, pharmaceuticals, sundry goods, and thelike.

1. A polylactic acid resin composition, comprising a resin composition(l) primarily including a poly-L-lactic acid having an L-form content of90 to 100 mol % and a resin composition (d) primarily including apoly-D-lactic acid having a D-form content of 90 to 100 mol %, the resincompositions (l) and (d) being stacked alternately one on another with(l) and (d) each being 0.01 to 2.5 μm in a single layer thickness,wherein a peak of a melting point is observed at 210° C. or higher inDSC measurement using a 10 mg sample, and an enthalpy of fusion ismeasured to be 60 J/g or more when the sample is heated to 280° C., heldfor 3 minutes, then immediately cooled rapidly, and further subjected toDSC measurement at a temperature raising rate of 20° C. per minute. 2.The polylactic acid resin composition according to claim 1, wherein theresin composition (l) and the resin composition (d) are stackedalternately one on another by making them pass a static mixer or amultilayer feed block.
 3. A polylactic acid resin drawn film produced byusing the polylactic acid resin composition according to claim 1 or 2,heating it to 280° C. or higher, then cooling it rapidly, and thendrawing it in at least one direction.